This invention relates to a process for separating C2-C3 olefins from industrial gases.
Many industrial processes produce olefin streams within the average carbon number range of C2-C3. The feedstock may be generated by commercial processes such as the olefinic (ethylene) products of hydrocarbon pyrolysis furnaces. Such hydrocarbon pyrolysis furnaces are typically fed by natural gas liquids, which contain significant amounts of ethane, propane, butane, or gasoline, or fed by naphtha or gas oil. Also the term xe2x80x9csteam cracker for production of olefinsxe2x80x9d is used. In some instances, operators do actually feed olefins to the cracking furnaces such as where they don""t have a market for C3, C4, C5 olefins, so the term xe2x80x9colefins steam crackerxe2x80x9d could be misunderstood, olefins normally not being a cracker feedstock. Other processes which generate feedstocks containing olefins include streams produced from the light ends (C4 or less) of any refinery thermal cracker using a vacuum flasher bottoms as the feed; a stream from the light ends (C4 or less) of any coker using a vacuum flasher bottoms as the feed; any one of the product streams from a fluidized catalytic cracking, (FCC) unit""s gas plant receiving the C4 or less cuts from a crude distillation column; the overhead of a cat cracking unit; or the C4 or less cut of a fractionation column fed by the product of a FCC Unit.
In many of these process, olefins are separated from impurities and by-products, such as saturated hydrocarbons, sulfur compounds, carbon oxides, and nitrogen containing compounds through costly cryogenic distillation, due to operating the process with expensive refrigeration units. Since the industrial C2-C3 streams are gaseous at room temperature, these streams are first cooled to liquify the ingredients followed by subjecting the liquid stream to a distillation process for separating the olefins from the liquid mixture. Another process to separate impurities, such as acetylenes, from ethylene streams is a solvent extraction. Solvent extraction, however, is not effective for separating olefins from saturated hydrocarbons. It would be desirable to separate out olefins from a wide variety of impurities and by-products, such as saturated hydrocarbons, carbon oxides, and sulfur and nitrogen bearing compounds, present in industrial streams containing olefins in the C2-C4 range, and which does not require the use of cryogenic units to conduct the separation.
U.S. Pat. Nos. 4,946,560, 5,936,136, and 5,942,656 describe processes for the separation of internal olefins from alpha olefins, and linear alpha olefins from 2-branched and/or 3-branched alpha olefins, by contacting a feedstock with an adducting compound such as anthracene or benzanthracene to form an olefin adduct, separating the adduct from the feedstock, dissociating the olefin adduct through heat to produce anthracene and an olefin composition enriched in alpha olefin, and separating out the anthracene from the alpha olefin. This reference does not suggest the desirability or the capability of anthracene to separate C2-C3 olefins contained in industrial streams from a gaseous mixture of C2-C3 olefins, saturated hydrocarbons, carbon oxides, sulfur bearing compounds, and nitrogen bearing compounds.
This invention relates to a process for treating a feedstock comprising olefins having an average carbon number ranging from, 2-3.5, and non-olefinic compounds, said process comprising the following steps:
a) contacting gaseous feedstock with a linear polyaromatic compound in a reaction zone under conditions effective to form a reaction mixture comprising linear polyaromatic compound-olefin adducts and unreacted gaseous feedstock;
b) separating the olefin adducts from the unreacted gaseous feedstock; and
c) dissociating the olefin adducts to form linear polyaromatic compounds and an olefin enriched composition comprising ethylene, propylene, or mixtures thereof;
whereby the concentration of at least one of the olefins in said olefin enriched composition is enriched over the concentration of the corresponding olefin in the feedstock.
As used throughout this specification and in the claims, the term xe2x80x9ccomprisingxe2x80x9d means xe2x80x9cat least,xe2x80x9d such that other unmentioned elements, ingredients, or species are not excluded from the scope of invention.
The feedstock to be treated comprises at least olefins and non-olefinic compounds, wherein the olefins have an average carbon number ranging from 2-3.5, based on a weighted average of moles of olefins present in the feedstock. An olefin means any compound containing at least one carbon-carbon double bond. Except for ethylene and propylene, other olefins present in the feedstock may be linear or branched. Examples of olefins which are present in the feedstock include at least one of ethylene, proplyene, or mixtures thereof or mixtures with other olefins such as butylene and/or pentenes.
The class of non-olefinic compounds generally includes saturated hydrocarbons. Saturated hydrocarbon include paraffins, and may also include other molecules such as cycloparaffins, which may be present in trace quantities. Other non-olefinic compounds typically present in the feedstock include methane, ethane, carbon dioxide, carbon monoxide, hydrogen, and sulfur and/or nitrogen bearing compounds. Examples of sulfur bearing compounds are H2S, alkyl mercaptans, CS2, and COS. Examples of nitrogen bearing compounds include ammonia, nitrogen, and amines.
The feedstock may be generated by commercial processes such as the product of an olefins (e.g. ethylene) pyrolysis furnaces; an olefins steam cracker; a stream from the light ends (mixtures of C4 hydrocarbons or less) of any refinery thermal cracker using a vacuum flasher bottoms as the feed; a stream from the light ends (mixtures of C4 hydrocarbons or less) of any coker using a vacuum flasher bottoms as the feed; any one of the product streams from a cat cracking gas plant receiving the mixture of C4 hydrocarbons or less cuts from a crude distillation column; the overhead of a cat cracking unit, and/or a catalyst reforming unit; any one of the products from the bottoms of a hydrotreater for removing sulfur compounds; the light butane overhead from a crude distillation column; the mixture of C4 hydrocarbons or less cut of a fractionation column fed by the product of a cat cracker, or derived by removal of light ends (C1-C4) in a Fisher-Tropsch product stream.
In one embodiment, the feedstock is a stream used to feed a C2 splitter fractionation column where ethylene is separated from ethane. The feedstock to a C2 splitter is derived from the product recovery and purification section of an olefins plant and originates in the radiant zone of the pyrolysis furnaces in the plant where feedstock pyrolysis takes place. Generally, the pyrolysis gases from the radiant zone of the ethylene furnace are separated first in a fractionator into gasoline and lighter fractions as an overhead and heavier than gasoline compounds as the bottoms in a fractionator. The overhead gases are compressed and cooled to form liquids and fed to a de-methanizer system to remove hydrogen and methane as an overhead. The remaining compounds from the bottoms of the de-methanizer are fed to a de-ethanizer to remove and isolate C2 compounds as an overhead for feedstock to the C2 splitter fractionation column which would be replaced by this invention. Other separation systems may have equipment arranged in a different sequence, or with different or additional equipment. In this embodiment, however, regardless of the separation system employed, the feedstock used for the separation system according to the invention is derived from an ethylene pyrolysis furnace which has been subjected to one or more separations to concentrate a stream containing C2 compounds, primarily ethylene and ethane. Most of the C2 feedstock streams contain at least 90 wt. % ethane and ethylene, and even at least 95 wt. % ethane and ethylene. Some of these streams contain at least 98 wt. % ethane and ethylene.
In another embodiment, the feedstock is a stream used to feed a C3 splitter fractionation column where propylene is separated from propane. The feedstock to a C3 splitter is derived from the product recovery and purification section of an olefins plant or a FCC Unit as the overhead stream of a depropanizer. Other product recovery and purifications systems may have equipment arranged in a different sequence such that the C3 stream is not the overhead stream of a depropanizer. In this embodiment, however, regardless of the separation system employed, the feedstock used for the separation system according to the invention is derived from an hydrocarbon pyrolysis furnace or an FCC process which has been subjected to one or more separations to concentrate a stream containing C3 compounds, primarily propylene and propane. The process of the invention is suitable to upgrade a C3 stream at a refinery by the separation techniques described below to a chemical (generally containing 92-95 wt % propylene) or polymer grade (containing at least 99.5 wt. % propylene), or to process a refinery grade propylene stream (containing about 60-80 wt. % propylene) off-site through the separation processes described below to a chemical grade or polymer grade propylene stream, or to process the C3 product stream from an steam cracker to produce either a chemical or polymer grade propylene stream.
The feedstock used in the invention has an average olefin carbon number ranging from 2-3.5, preferably from 2-3, inclusive. The feedstock used in the invention contains at least one of ethylene, propylene, or mixtures thereof. In another embodiment, the feedstock comprises ethylene, ethane, hydrogen, and carbon dioxide. In yet another embodiment, the feedstock comprises methane, ethane, ethylene, propylene, carbon dioxide, carbon monoxide, hydrogen, and sulfur and/or nitrogen bearing compounds.
The amount of each ingredient varies widely depending upon the source of the feedstock. In one embodiment, the amount of any one of ethylene and propylene are independently at least 5% by volume, preferably at least 10% by volume. In another embodiment, the feedstock comprises, by volume percentage based on the volume of the feedstock, 5-35% ethylene, 0.0-20% and preferably 1% to 20% propylene, 0.0 to 25% and typically 0.1-25% carbon dioxide, 0.0% to 10% and typically 0.5-10% carbon monoxide, 0.0 to 40% and typically 5-40% hydrogen, and from 0.0% to 4% and typically 0.0005-1% sulfur bearing compounds. In yet another embodiment, the volume percentage of the ingredients in the feedstock comprises from 20-60% methane, 10-30% ethane, 5-35% ethylene, 0.5-20% propylene, 0.1-25% carbon dioxide, 0.5-10% carbon monoxide, 5-40% hydrogen, and from 0.0005-1% sulfur bearing compounds.
The linear polyaromatic compound is utilized in the instant process to form the adduct with the olefins in the feedstock. As used herein, xe2x80x9clinear polyaromatic compoundxe2x80x9d refers to a linear polyaromatic compound having at least three fused aromatic rings, which may be unsubstituted or substituted and possess similar adducting properties as the unsubstituted molecule, and mixtures thereof. The linearity should extend to all three of the fused rings if a three fused ring compound is used and to at least four consecutively fused cyclic rings if a four or more fused ring compound is used. The linear polyaromatic compound also refers to mixtures of compounds containing as one of their ingredients the linear polyaromatic compound, including but not limited to coal tars, anthracene oil, and any crude mixtures containing cuts separated from naphthalene. The linear polyaromatic compound also includes aromatic molecules linked together by a bridging group, such as a-hydrocarbon chain, an ether linkage, or a ketone group containing chain so long as at least three fused rings are present in a linear arrangement; as well as those containing a heteroatom which do not interfere in the separation of olefins from saturated hydrocarbons.
Non-limiting examples of the linear polyaromatic compound include anthracene, 2,3-benzanthracene, pentacene, and hexacene. Suitable examples of substituents on substituted linear polyaromatic compounds include, but are not limited to, lower alkyl, e.g., methyl, ethyl, butyl, isopropyl; halo, e.g., chloro, bromo, fluoro; nitro; sulfato; sulfonyloxy; carboxyl; acetyl; carbo-lower-alkoxy, e.g., carbomethoxy, carbethoxy; amino; mono- and di-lower-alkylamino, e.g., methylamino, dimethylamino, methylethylamino; amido; hydroxy; cyano; lower-alkoxy, e.g., methoxy, ethoxy; lower-alkyanoyloxy, e.g., acteoxy; monocyclic aryls, e.g., phenyl, xylyl, toluyl, benzyl, etc. The particular substituent size, their number, and their location, should be selected so that they are relatively inert under the reaction conditions and not so large as to block the formation of the Diels-Alder adduct. Suitable substituted linear polyaromatic compounds can be determined by routine experimentation. Examples of suitable linear polyaromatic compounds include 9,10-dimethylanthracene, 9,10-dichloroanthracene, 9-methylanthracene, 9-acetylanthracene, 9-(methylaminomethyl)anthracene, 2-choloranthracene, 2-ethyl-9,10-dimethoxyanthracene, anthrarobin, and 9-anthryl trifluoromethyl ketone. The preferred linear polyaromatic compounds are anthracene and 2,3-benzanthracene. In another preferred embodiment, the linear polyaromatic compound comprises anthracene having a purity of 75% or more anthracene. The selectivity of the linear polyaromatic compound toward the olefins in the feedstock of 80% or more, preferably 90% or more, more preferably 95% or more, and most preferably 98% or more, and even 99% or more.
The conversion of the olefins in the feedstock should be sufficiently high to economically justify removal of the olefins from the feedstock. In one embodiment, at least 30 %, more preferably at least 40%, of the olefins in the feedstock are converted, depending upon the desired residence time and operational temperatures and pressure.
In a first reaction zone in step a), the feedstock composition having an average olefin carbon number ranging from 2-3.5 is contacted with a linear polyaromatic compound. The resulting products of the reaction are Diels-Alder adducts. Thus, there is provided an adduct of a linear polyaromatic compound and ethylene in one embodiment. In another embodiment, there is provided an adduct of a linear polyaromatic compound and propylene. In each of these embodiments, the preferred linear polyaromatic compound is anthracene, although any of the above identified linear polyaromatic compounds can be used.
The Diels-Alder adduct forming reaction is carried out in a conventional fashion. Examples of suitable equipment in which the reactions are carried out include a continuously stirred tank reactor, configured as a single unit, in parallel, or in series, wherein feedstock or an portion of the olefin composition, and linear polyaromatic compound, are added continuously to a stirred tank to form a liquid reaction mixture under heat, and the reaction mixture is continuously withdrawn from the stirred tank. Alternatively, the reaction may be carried out in a plug flow reactor or a series of plug flow reactors, a bubble column, or in a batch reactor.
The reactions can be carried out by reacting a gaseous feedstock (gaseous under adducting reaction conditions)with the linear polyaromatic compound in the liquid or gaseous phase. In a gas-gas phase, the system pressure should be lowered to sub-atmospheric pressure to volatize the linear polyaromatic compound at a temperature sufficiently low to maintain a stable adduct. In a preferred embodiment, the reaction is carried out by contacting the feedstream in a gaseous phase at reaction conditions with a liquid phase linear polyaromatic compound. In this preferred embodiment, the gaseous feedstock may react to form the adduct upon contact with the liquid linear polyaromatic compound, or the gaseous feedstock may first dissolve into the liquid linear polyaromatic compound followed by formation of the adduct, or both reactions may occur simulteneously.
The method of contact is not limited. Examples of contact include spraying the linear polyaromatic compound into a continuous phase of the gaseous feedstock, or bubbling a gaseous feedstream through a continuous phase of liquid linear polyaromatic compound.
The adducting forming reactions are typically carried out over a range of temperatures depending upon the pressure. Examples of suitable temperatures in this embodiment range from about 150xc2x0 to about 375xc2x0 C., more preferably from about 230xc2x0 to about 320xc2x0 C., and most preferably from about 240xc2x0 to about 310xc2x0 C. In general, the adducting reaction can be carried out under any pressure, but it is preferred to conduct the reaction at elevated pressure to increase the reaction rate. Preferred system pressures range from 50 psig to 1000 psig, and more preferably from 200 to 350 psig.
In the mixed liquid linear polyaromatic compound/olefin gas phase embodiment, the reactor system temperature/pressure is within a range which maintains the linear polyaromatic compound in liquid state without liquifying incoming feedstock in the gas phase and without dissociating the adducts as formed or preventing their formation. Preferred reactor temperatures range from 200-300xc2x0 C., more preferably from 250-280xc2x0 C. The reactor system pressure in this embodiment ranges from 0.0 psig to 1000 psig, more preferably from 50 to 500 psig.
The residence time is for a time sufficient to adduct the desired amount of linear polyaromatic compound with the olefin. The optimal time will be the shortest time to attain the target conversion of olefins at the designed reaction temperature and pressure.
An inert solvent can be utilized to dissolve the feedstock olefins or the linear polyaromatic compound or both in each of the adducting reactors. Preferred solvents are the hydrocarbon solvents which are liquid at reaction temperatures and in which the olefins, linear polyaromatic compound and olefin-linear polyaromatic compound adducts are soluble. Illustrative examples of useful solvents include the alkanes such as pentane, iso-pentane, hexane, heptane, octane, nonane, and the like; cycloalkanes such as cyclopentane, cyclohexane, and the like; and aromatics such as benzene, toluene, ethylbenzene, diethylbenzene, and the like. The amount of solvent to be employed can vary over a wide range without a deleterious effect on the reaction.
Preferably, the adducting reactions are carried out in the absence of a solvent, thereby improving the rate of reaction and avoiding the need for additional equipment and process steps for separating the solvent.
After formation of the linear polyaromatic compound-olefin adduct in step a), the adduct stream (optionally and generally including unreacted liquid linear polyaromatic compound) is separated from the unreacted gaseous feedstock in a step b). Step a) and step b) may be, and typically will be, carried out in the same reaction vessel used to conduct the adducting reaction. Further, step b) may be, and typically will be, carried out simultaneously with step a) as the adduct is forming. For example, in a bubble column, as the gaseous feedstock is contacted with a continuous phase of liquid linear polyaromatic compound, adducts of linear polyaromatic compound-olefins are formed while the unreacted gaseous feed continues up through the liquid phase and is removed as a gaseous overhead. The adducts and liquid linear polyaromatic compound are simultaneously removed as the unreacted gases are taken at the overhead. In a spray process, the liquid linear polyaromatic compound may be sprayed into a gaseous feedstream, thereby simultaneously forming the adduct, which along with unreacted linear polyaromatic compound, is removed from any unreacted gaseous feedstock which is removed as an overhead.
The recovery of the linear polyaromatic compound-olefin adducts is determined by the molar ratio of linear polyaromatic compound to olefins, the adducting residence time, the temperature within the reactor vessel, and most importantly, the residence time (rate of separation) of the reaction mixture in the reactor vessel. To obtain a large olefin composition recovery, a high linear polyaromatic compound to olefin molar ratio, e.g.,  greater than 100, and long residence times to ensure complete adduction.
In the next step of the process, the removed linear polyaromatic compound-olefin adduct stream is dissociated in step c) in a dissociation zone to form linear polyaromatic compounds and an olefin enriched composition. The dissociation process can be accomplished by feeding the adducted olefin stream to a dissociation vessel where the adducted olefin stream is heated and pyrolyzed at a temperature ranging from about 200xc2x0 to about 500xc2x0 C., preferably from about 300xc2x0 to about 350xc2x0 C., for a time sufficient to dissociate the adducts, thereby releasing the olefin gases from the liquid phase linear polyaromatic compounds. The dissociation temperature can be further reduced below 200xc2x0 C. by drawing a vacuum on the dissociation vessel. The pyrolysis frees the olefins from the linear polyaromatic compound. One or more dissociation vessels may be used in series to conduct the dissociation.
In an optional but preferable step d), trace or small quantities of linear polyaromatic compound entrained in the released olefin enriched composition may be removed by adsorption with cold linear polyaromatic compound or by scrubbing the gaseous olefin enriched composition with any other appropriate sorbents. Optionally, trace or small quantities of linear polyaromatic compound entrained in the released olefin enriched composition may be removed by simply cooling the gaseous olefin enriched composition, or by flash distillation, thereby removing the olefins from the linear polyaromatic compound.
The linear polyaromatic compounds may be recycled back to the adducting reaction zone or to a mixing zone wherein the linear polyaromatic compound recycle and some fresh linear polyaromatic compound, along with optional feedstock, are premixed prior to entering the adduct reaction zone.
The olefin enriched composition is enriched in the concentration of olefins over the concentration of olefins in the feedstock.
For purposes of measuring the % enrichment of a species in a stream, the concentration of the species or series of species in the predecessor or feedstock stream is subtracted from the concentration of species or series of species in question contained in the product stream, the difference then divided by the concentration of those same species present in the predecessor feedstock stream and multiplied by 100.
In another embodiment of the invention, the concentration of saturated hydrocarbons in the olefin enriched composition is reduced through the process of the invention in only one pass by at least 80%, preferably by at least 90%, more preferably by at least 95% over the concentration of saturated hydrocarbon in the feedstock, and most preferably by.98% or more.
The concentration of olefins in the olefin enriched composition is enriched over the concentration of olefins in the feedstock. The degree of olefin enrichment varies inversely with the concentration of olefins present in the feedstock. In a preferred aspect of this embodiment, the concentration of olefins in the olefin enriched composition is enriched by at least 40%, preferably by at least 60%.
The resulting olefin enriched composition may be used as a feedstock for the manufacture of higher olefins in the C4-C100 range, polyethylene, polypropylene, propylene oxide and derivatives such as propylene glycol, normal butyl alcohol, ethylene oxide and derivatives such as ethylene glycol, ethylbenzene, cumene, isopropyl alcohol, propanol, ethanol, epichlorohydrin, acetone and its derivatives such as MIBK, oligomerization products using a Dimersol(trademark) process or a Polygas(trademark) process, and in any other process which employs ethylene and/or propylene as a feedstock material.